The invention relates to a flat display device using an electrical beam. More specifically the invention relates to a flat display device having a plurality of control electrodes coated with resistivity films on their surfaces.
FIG. 2 is a perspective view showing a part of the conventional flat display device disclosed in the laid-open Japanese patent publication No. 63-184239/1988. In FIG. 2, 1 is a linear hot cathode which emits the electrons by the current flowing through it. 3 is a cover electrode having holes on the surface, which shape is, for example, a part of an ellipse. The cover electrode is arranged so that it covers the linear hot cathode 1 and attracts and accelerates the hot electrons which are generated from the hot cathode 1. The cover electrode 3 has many small holes 1a on its surface, and attracts the hot electrons 2 from the linear hot cathode by applying appropriate electrical potential. 8 is a front glass which is coated by dot shape fluorescent materials. The dot shape fluorescent materials form a fluorescent body 9. The fluorescent body 9 is excited by the electron 2 and generates red, green and blue light. A conductive aluminum film (not shown) is formed on the surface of the fluorescent body 9. The electron 2 is accelerated by the voltage of about 5-30 kV applied to the aluminum film, and causes the fluorescent body 9 to excite and to generate light.
4 is a control electrode which is arranged between the front glass 8 and linear hot cathode 1 and also arranged substantially parallel to the linear hot cathode 1. The control electrode 4 controls the emitted electron beam, which is attracted by the cover electrode 3 and directed to the front glass 8, so that the beam can pass through or can not pass through the control electrode 4. The control electrode 4 consists of an insulated substrate 5, metal electrodes 6 and metal electrodes 7. 20 is a back electrode arranged to the opposite side of the cover electrode 3 against the linear hot cathode 1.
FIG. 3 is an exploded view of the control electrode 4. The insulated substrate 5 has electron pass holes 5a corresponding to the picture elements on the front glass 8. Strap-shaped metal electrodes 6 are arranged under the insulated substrate 5 corresponding to each column of the picture element. Each strap-shaped metal electrodes 6 have electron pass holes 6a corresponding to the picture elements. The metal electrodes 6 consist of a first control electrode group. In the same way, strap-shaped metal electrodes 7 are arranged over the insulated substrate 5 corresponding to each row of the picture elements. Each strap-shaped metal electrode 7 has electron pass holes 7a corresponding to the picture elements. The metal electrodes 7 consist of a second control electrode group. The first control electrode group 6 and the second control electrode group 7 are bonded so that the electron pass holes 6a and 7a are aligned with the electron pass holes 5a of the insulated substrate 5.
The operation of the invention is explained below. The electrons 2 emitted from the linear hot cathode 1 are attracted to the cover electrode 3 by the plus electric potential of about 2-20 volts applied to the cover electrode 3. Further, the electrons are attracted and reach the control electrode 4 by applying the plus electrical potential of about 20-50 volts to one of the electrodes of the first control electrode group 6 which is perpendicular to the linear hot cathode 1, against the linear hot cathode 1. The electron beam density is controlled to be homogeneous at the front surface of the metal electrode of the first control electrode group 6 by regulating the elliptic shape of the cover electrode 3, the position of the first control electrode group 6 and the voltage applied to each metal electrode 6.
FIG. 4 is an illustration showing a movement of the electrons attracted from the cover electrodes 3. In FIG. 4, the electrons 2 do not always enter into the control electrode vertically, since each electron has different initial velocity when it is attracted from the cover electrode 3. Therefore, some electrons 2a enter vertically into the control electrode 4 and some electrons 2b enter obliquely into the control electrode 4.
The operation of the control electrode 4 is not described in the laid-open patent publication No. 63-184239/88, but it is described in detail in the laid-open patent publication No. 62-172642/86 or No. 2-126688/90.
In FIG. 3, if the plus electric potential is applied to one of the control electrode group 6 and minus electric potential is applied to the other control electrode group 6, the hot electrons emitted from the linear hot cathode are attracted to only one of the metal electrode and pass through each electron pass hole and enter into the electron pass hole 5(a) of the insulated substrate 5. But all electrons entered into the electron pass hole 5a do not always pass through to the front glass 8.
FIG. 5 is an illustration showing a movement of the electrons passing through the control electrode 4. In FIG. 3, electrodes are not formed on the inner wall surface of the electron pass hole 5a. But in FIG. 5, the electrodes are formed on the inner wall surface of the electron pass hole 5a. In FIG. 5(a), the second control electrode 7x is formed on the surface of the substrate 5 at the wall of the electron pass hole 5a. Since zero volts or minus volts are applied to the second control electrode 7x, the negative potential area 10 is formed in the electron pass hole 5a. Therefor, the electrons 2 stop in the electron pass hole 5a. In FIG. 5(b), plus voltage is applied to the second control electrode 7x. The electrons which enter vertically into the substrate 5 pass through the electron pass hole 5a. But some electrons which enter obliquely into the electron pass hole 5a hit the substrate 5 and charge up the substrate 5, because a part of the substrate is exposed to the wall surface of the electron pass hole 5a.
FIG. 6 is an illustration showing a movement of the electrons passing through the control electrode 4. In FIG. 6(a), the electrons 2 pass through the electron pass hole 5a when the voltage of 40 to 100 volts are applied to the second control electrode 7 arranged on the top surface of the electron pass hole 5a. But as shown in FIG. 6(b), some electrons hit the exposed insulated substrate 5 and charge up the insulated substrate 5 if the electrons enter obliquely to the control electrode 4.
From FIG. 5 and FIG. 6, it is understood that the electrons can pass through the cross point where the plus electrical potential is applied to both the first control electrode 6 and the second control electrode 7. The electrons passed through the control electrode 4 hit the picture elements on the fluorescent body 9 corresponding to the cross points. Then, the fluorescent body 9 generates light and causes the picture on the display. Therefore, a desired picture is obtained by controlling the voltage applied to each metal electrode 6 and 7 corresponding to the desired cross points.
It is necessary that the control electrode 4 interrupts the electron beam to pass through when the small minus voltage is applied to the control electrode 4, or the control electrode 4 causes the electron beam to pass through when the appropriate plus voltage is applied to the control electrode 4. To achieve the above controlling feature, the control electrode 4 must be formed by an appropriate shape.
As described above, since the prior art flat display device is constructed of the strap-shaped electrode having the first control electrodes arranged in a column and the second control electrodes arranged in a row, it is difficult to bond the two types of strap-shaped electrodes which are separately manufactured. The most actual resolving method is to manufacture the control electrode 4 using a general printed wiring substrate. For example, one of the method for manufacturing the control electrode 4 is to form the conductive thin film on the surface of the insulated substrate 5 and on the inner wall surface of the electron pass hole 5a by a plating process, and then to eliminate the thin film at the desired position by an etching process.
FIG. 7 is one of the prior art manufacturing methods of the control electrode disclosed in the laid-open patent publication No. 58-46562/81, which construction is explained below. As shown in FIG. 7(a) and FIG. 7(b), at first the conductive films 43 and 53 are formed on the insulated substrate 41 and 51, respectively, then the electron pass holes 42 and 52 are formed in a row or column, respectively, and then the conductive films are formed on the inner wall surfaces of the electron pass holes, respectively. As shown in FIG. 7(c), the two substrates are bonded by the insulated materials 61 and 62 which function as insulated spacers. FIG. 7(c) shows a sectional view at A--A line of FIG. 7(a) and FIG. 7(b). As shown in FIG. 7, in the prior art construction of the control electrode, since the insulated spacers 61 and 62 are exposed at the inner wall of the electron pass hole, the insulated spacers 61 and 62 are charged by the incoming electrons. The charged insulated material existing near the electron pass hole causes many harmful effects to the display device as shown below.
First of all, the intensity of the display degrades. FIG. 8 is an illustration showing a movement of the electrons passing through the control electrode 4. As shown in FIG. 8(a), when the insulated substrate 5 is charged, the minus potential area 10 is formed by the negative charge 11 stored at the surface of the insulated material. Therefore, the area where the electrons pass through is substantially narrowed, and the current beam decreases at the electron pass hole even if the hole aperture is the same. Accordingly the intensity of the display screen degrades.
We made two control electrodes in which the exposure distance d of the insulated material shown in FIG. 8(a) is 100 .mu.m (board thickness 600 .mu.m) and 50 .mu.m, respectively, using free-cutting ceramic substrate and conductive electrode deposited by Ni. The result of the comparison with the two model control electrodes showed about ten times difference regarding the screen intensity (candela conversion) under the same condition. For degrading the influence of the charge, it is able to apply the high voltage to the electrodes 6 and 7. But, in order to obtain a dynamic screen, it is necessary to apply a signal to the electrodes 6 and 7 at least several kHz. Considering the application to the mass production goods such as a television set, to apply a high voltage to the control electrode is not a good method.
Second, the operation of the display screen is not stable. More specifically, since it takes a lot of time until the charge quantity becomes a predetermined value, it takes a lot of time until the display screen operates in a comparatively stable state after closing the switch of the display device. It took about several tens of minutes until the above model electrode (exposure distance d=100 .mu.m) had operated in a stable state. After the time, there occurred many irregular discharges from the charged insulated material at every place in the electrode and also occurred the flicker in the display screen.
In the laid-open patent publication No. 58-46562/81, in order to avoid the harmful influence of the charge up of the above insulated material, the resolving idea is described where spacers are arranged so as to be retracted from the inner wall of the electron pass hole. But as long as the insulated materials are exposed in the inner wall, it is very difficult to avoid the influence of the charge completely.
As already described in FIG. 4 and FIG. 5, the incidence of the electrons to the surface of the electron pass hole can not be avoided, since there is a velocity component toward the radial direction of the electron pass hole of the electron. Since the degree of vacuum of the vacuum part of the electron picture display device is about 10.sup.-7 Torr, for example, in the case of the television set, therefore, it is very difficult to cause the electrons to discharge from the charged insulated material through the vacuum part.
Even if the harmful influence is avoided by arranging the insulated substrate 5 so as to be retracted from the inner wall of the electron pass hole, it is very difficult to actually mass-produce the control electrode 4 having such construction. FIG. 9 is an enlarged sectional view of the prior art control electrode. FIG. 9(a) is a top view of the control electrode 4. FIG. 9(b) is a B--B line cross sectional diagram of FIG. 9(a).
In the figure, the actual manufacturing of the control electrode is described below. Assume that the diameter of the picture element is 0.6 mm, the diameter of the electron pass hole is 0.4 mm, the retracted distance from the inner wall surface of the electron pass hole is over 50 .mu.m, the arranging range (indicated in W) of the insulated substrate 5 is only 100 .mu.m. It is very difficult to manufacture the insulated substrate 5 within the above range in good yield and in good accuracy through the all area of the screen (about 20 inch square) of the television set. The largest reason of the difficulties is in that picture elements amount to about 300,000 through the entire area of the 20 inch display screen. Only one of the defective picture elements degrades a commercial value of the display device.
In order to decrease the harmful influence generated by charging the insulated substrate, it is able to shorten the exposure distance d of the insulated material as shown in FIG. 8(a). But, in order to neglect the harmful influence, the exposure distance d of the insulated material must be narrower than several tens .mu.m. But, in case of very narrow exposure distance, the insulation between the upper electrode 6 and the lower electrode 7 will deteriorate. Therefore, the exposure distance must be formed accurately within the predetermined range lower than several tens .mu.m on the inner wall having the hole depth (=substrate thickness) of several hundreds .mu.m. As described above, it is also very difficult to manufacture the insulated substrate 5 within the above range in good yield for all picture elements of about 300,000.